For example, US2005/0103124A1 (JP-A-2005-172793) discloses a compression-type load detection element, which includes a pressure-sensitive element and electrical insulators. The pressure-sensitive element has an electrical property, which changes in response to stress applied thereto. The pressure-sensitive element has two opposed surfaces on which the electrical insulators are integrally provided. The pressure-sensitive element of the load detection element includes a matrix, which is formed from glass, and conductive particles, which have conductivity. The conductive particles are distributed in the matrix. The conductive particles are formed from RuO2, for example. The thickness of the pressure-sensitive element is 1 to 200 micrometers, for example. The pressure-sensitive element is provided with a pair of electrodes.
As disclosed in US2005/0193828A1 (JP-A-2005-249598) and JP-A-11-37860, the load detection element is capable of detecting compression load in addition to tensile load by applying preload to the load detection element in advance. US2005/0193828 A1 and JP-A-11-37860 may teach an effect produced by applying the preload to the load detection element, nevertheless fails to teach a method for applying the preload in detail. The preload within a preferable range needs to be applied, and therefore the preload needs to be adjusted one by one in consideration of dimensional variation of components of the load detection element. In general, torsion strength of the load detection element is less than compression strength thereof, and therefore the load detection element is less durable against application of torque compared with application of compression load. Therefore, in the structure of US 2005/0193828A1 and JP-A-11-37860, in each of which the screw is used to adjust the preload, the load detection element may be applied with torque in response to application of the preload. In general, the rigidity in the torsion direction of the load detection element is insufficient relative to rigidity in the compression direction. Since load detection element may be broken when being applied with excessive torque, the preload caused by applying torque to the load detection element may be limited so as not to cause failure in the load detection element.
An example of a load sensor will be described with reference to FIG. 6. A load sensor 91 includes a load detection element 910, a first structural member 920, a second structural member 930, a preload adjusting member 935, a signal processing circuit 950, and the like. The preload applied to the load detection element 910 is adjusted by screwing the preload adjusting member 935 into the second structural member 930. In order to increase the preload, the preload adjusting member 935 needs to be further screwed. However, stress applied from the preload adjusting member 935 to the load detection element 910 increases in response to increase in screwing depth and application of preload. Consequently, friction between the preload adjusting member 935 and the load detection element 910 increases, and therefore the torque is apt to be further transmitted to the load detection element 910. As the preload adjusting member 935 is further screwed, the torque is further directly transmitted to the load detection element 910. When large preload is applied to the load detection element 910, excessive torque may be applied to the load detection element 910. Therefore, maximum preload is limited in consideration of the torque applied to the load detection element 910.
As described above, the preload may not be sufficiently applied in the screwing structure, in which the preload adjusting member 935 is directly onto the load detection element 910. In addition, in the screwing structure, the load applied to a detected object is in series with the load detection element when the preload adjusting member is screwed to the second structural member, and consequently the screwed portion is apt to cause elastic deformation. Therefore, the load is apt to be consumed by elastic deformation in the screwed portion, compared with the structure in which the load is directly applied to the load detection element. Thus, external tensile and compression load is less applied to the load detection element in the screwing structure. Consequently, an assignment of the load applied to the load detection element becomes small, and the sensitivity of detection of load may become insufficient.